46
3 An ERA Acute Model Overview
where:
• : cumulative normal distribution function: with μ = 0 and σ = 1
• x: target THC concentration.
3.5.1.2 Externally Calculated Lethal Fraction
In addition to the complexity of a three-dimensional compartment and varying
composition of the spilled oil due to weathering processes, the main challenges for
computing the impact of oil to water column organisms include the temporal variation
in toxicity of the oil, as well as temporal and spatial variations in oil concentrations
due to transport and weathering. This is better reflected when the potential mortality
accumulates during the course of the oil drift simulations and requires access to an
oil drift model that calculates an accumulated fraction of the eggs/larvae that are
killed. This fraction is then used directly as p let . ERA Acute allows for the results
of advanced oil spill models that calculate the eggs/larvae fraction lost to be entered
into the model but does not require it. Below, a description is given on how the oil
spill model OSCAR calculates this potential fraction killed.
Oil in the water column is partitioned between dispersed oil droplets and watersoluble fractions (dissolved oil components). For the dissolved phase, the “fraction
killed” per cell is accumulated over the time steps of the simulation using a Quantitative Structure-Activity Relationship (QSAR) between toxicity and the composition
and amount of the dissolved hydrocarbons at the time step. Choice of reference oil is
therefore an important driver in the result, as different oils have different hydrocarbon
group compositions. Based on their molecular structure, the toxicity of the dissolved
phase is calculated and the toxicity of the mix is a function of the composition of
the hydrocarbon mix, as known from QSAR theory used in predictive toxicology
(French-McCay 2002). This approach uses the octanol-water partitioning coefficient
(K OW ) and the corresponding narcotic effect as the endpoint.
Time- averaged concentration and the corresponding mean composition are calculated for the actual exposure times (τ) in subsequent 96-hour periods. The exposure
time is defined as the time when dissolved oil is present at a concentration > 0 in the
given 96-hour period (Johansen et al. 2005).
Each component group has an LC50 value and at each time-step (in each cell)
the corresponding potential lethality of the mix is calculated by a modification of
Eq. 3.11 (French-McCay 2002).
LC50 mix = 1
F j
LC50 j
(3.11)
where F is the fraction of the component j in the mix. The modification adjusts for
exposure time (τ) by the following equation (Johansen et al. 2005);
LC50(τ ) = LC50 ∞
1 − exp(−ετ )
(3.12)
3 An ERA Acute Model Overview
where:
• : cumulative normal distribution function: with μ = 0 and σ = 1
• x: target THC concentration.
3.5.1.2 Externally Calculated Lethal Fraction
In addition to the complexity of a three-dimensional compartment and varying
composition of the spilled oil due to weathering processes, the main challenges for
computing the impact of oil to water column organisms include the temporal variation
in toxicity of the oil, as well as temporal and spatial variations in oil concentrations
due to transport and weathering. This is better reflected when the potential mortality
accumulates during the course of the oil drift simulations and requires access to an
oil drift model that calculates an accumulated fraction of the eggs/larvae that are
killed. This fraction is then used directly as p let . ERA Acute allows for the results
of advanced oil spill models that calculate the eggs/larvae fraction lost to be entered
into the model but does not require it. Below, a description is given on how the oil
spill model OSCAR calculates this potential fraction killed.
Oil in the water column is partitioned between dispersed oil droplets and watersoluble fractions (dissolved oil components). For the dissolved phase, the “fraction
killed” per cell is accumulated over the time steps of the simulation using a Quantitative Structure-Activity Relationship (QSAR) between toxicity and the composition
and amount of the dissolved hydrocarbons at the time step. Choice of reference oil is
therefore an important driver in the result, as different oils have different hydrocarbon
group compositions. Based on their molecular structure, the toxicity of the dissolved
phase is calculated and the toxicity of the mix is a function of the composition of
the hydrocarbon mix, as known from QSAR theory used in predictive toxicology
(French-McCay 2002). This approach uses the octanol-water partitioning coefficient
(K OW ) and the corresponding narcotic effect as the endpoint.
Time- averaged concentration and the corresponding mean composition are calculated for the actual exposure times (τ) in subsequent 96-hour periods. The exposure
time is defined as the time when dissolved oil is present at a concentration > 0 in the
given 96-hour period (Johansen et al. 2005).
Each component group has an LC50 value and at each time-step (in each cell)
the corresponding potential lethality of the mix is calculated by a modification of
Eq. 3.11 (French-McCay 2002).
LC50 mix = 1
F j
LC50 j
(3.11)
where F is the fraction of the component j in the mix. The modification adjusts for
exposure time (τ) by the following equation (Johansen et al. 2005);
LC50(τ ) = LC50 ∞
1 − exp(−ετ )
(3.12)